The present invention relates generally to laser diode sources, and more particularly, to controlled-linewidth laser diode sources.
Single-mode semiconductor lasers are generally used as the light sources in optical fiber transmission systems and optical fiber sensors. Accordingly, the spectral characteristics of the single-mode semiconductor lasers are important in that they set the limits on the performance capabilities of these optical fiber transmission systems and sensors. Narrow spectral linewidth and low frequency wavelength stability are particularly vital in optical heterodyne communication systems and interferometric fiber sensors having large path length differences.
One approach to line narrowing is to simply increase the output power of the laser by driving it harder with more electrical current. The linewidth has been shown to go from 100 MHz to less than 10 MHz when the output of the laser was changed from 1 mW to 10 mW. ("Fundamental Line Broadening of Single-Mode (GaAl) As Diode Lasers," Mark W. Fleming and Aran Mooradian, Applied Physics Letters, 38, (7), Apr. 1, 1981, pages 511-513). However, the problem with this approach is that the higher drive currents reduce the laser lifetime and may even destroy the laser completely. Furthermore, the reduction in linewidth using this method is not sufficient for many applications.
An alternative method is based on the fact that changes in the spectral characteristics of a single-mode laser occur when a portion of the laser output is fed back into the laser cavity. A number of investigators ("Direct Observation of Lorentzian Lineshape of Semiconductor Laser and Linewidth Reduction with External Grating Feedback," S. Saito and Y. Yamomoto, Electronic Letters, Vol. 12, No. 9, Apr. 30, 1981, pages 325-327; "Heterodyne-type Optical Fiber Communications," T. Okoshi, Proceedings of the Third International Conference on Integrated Optics and Optical Fiber Communications, paper TUB 1, pages 44-45; and "Laser Diode Emitter For Heterodyne-Type Communication Systems," F. Fabre and D. LeGuen, Proceedings of the Third International Conference on Integrated Optics and Optical Fiber Communication, paper MJ 3, page 34) have been able to narrow the spectral linewidth of a laser diode by reflecting a portion of the optical output signal back into the laser diode cavity. The reflecting element used was either a mirror, a diffraction grating, or an optical fiber. These studies have found that with no reflection or feedback into the laser cavity so that the laser operates in a free running mode, the frequency linewidth of commercially available lasers is typically 5 MHz or larger, corresponding to a wavelength linewidth of 1.0.times.10.sup.-4 angstroms. However, in the presence of optical feedback, frequency linewidths of less than 100 kHz have been observed.
A prior art embodiment utilizing this reflection feedback technique is shown in FIG. 1. This prior art design comprises a laser diode 10 in combination with a reflecting element 20. The laser diode 10 is powered by a power supply 12 capable of providing 20-100 mA in order to inject charge carriers into the laser cavity. The laser 10 generates coherent radiation under proper biasing conditions. This coherent radiation is directed through one facet of the semiconductor laser toward the reflecting element 20. In order to control the amount of light being reflected back into the laser cavity, a neutral density filter 14 may be placed between the laser cavity 10 and the reflecting element 20. Such a neutral density filter generally comprises a glass slide covered with varying amounts of metal. By positioning the slide in the optical beam, the amount of light transmitted and consequently reflected is controlled. Because the active layer in a semiconductor laser is typically very narrow in one dimension (on the order of less than a micron), the optical output from this active layer is highly diverging. Accordingly, for most applications a collecting lens 16 is used to collimate the beam 18. Since a typical laser cavity has two mirrors or end facets, one facet may be used to provide a desired output, while the other facet may be used to provide the linewidth control reflections. Reflections of light back through the output end facet can be eliminated with a Faraday isolator, so that linewidth control can be accomplished by reflecting through only one facet of the laser.
There are a number of difficulties that arise from this prior art reflection configuration. One of the foremost difficulties is that this configuration provides no precise control over the amount of light being reflected back into the laser cavity. Additionally, mechanical alignment of the laser facets, the density filter, and the reflecting element have a significant effect on the amount of light being reflected. Thus, precision linewidth control is difficult to achieve. Furthermore, the overall size of the device is relatively large compared to a conventional laser diode.